August 31, 2000
01:00 AM (EDT)

News Release Number: STScI-2000-24

He 2-90's Appearance Deceives Astronomers

August 31, 2000: Astronomers using NASA's Hubble Space Telescope have stumbled upon a mysterious
object that is grudgingly yielding clues to its identity. A quick glance at the
Hubble picture at top shows that this celestial body, called He 2-90, looks like
a young, dust-enshrouded star with narrow jets of material streaming from each
side. But it's not. The object is classified as a planetary nebula, the glowing
remains of a dying, lightweight star. But the Hubble observations suggest that
it may not fit that classification, either. The Hubble astronomers now suspect
that this enigmatic object may actually be a pair of aging stars masquerading
as a single youngster. One member of the duo is a bloated red giant star shedding
matter from its outer layers. This matter is then gravitationally captured in
a rotating, pancake-shaped accretion disk around a compact partner, which is most
likely a young white dwarf (the collapsed remnant of a sun-like star). The stars
cannot be seen in the Hubble images because a lane of dust obscures them.

Q & A: Understanding the Discovery

1.
What does the top image show?

The Hubble picture at top shows a centrally bright object with jets, appearing like strings of beads, emanating from both sides of center. (The other streaks of light running diagonally from He2-90 are artificial effects of the telescope's optical system.) Each jet possesses at least six bright clumps of gas, which are speeding along at rates estimated to be at least 375,000 miles an hour (600,000 kilometers an hour). These gaseous salvos are being ejected into space about every 100 years, and may be caused by periodic instabilities in He2-90's accretion disk.

An accretion disk needs gravity to form. For gravity to create He2-90's disk, the pair of stars must reside at a cozy distance from each other: within about 10 astronomical units (one astronomical unit equals the Earth-Sun distance, 93 million miles). Although the astronomers are uncertain about the details, they believe that magnetic fields associated with the accretion disk produce and constrict the pencil-thin jets seen in the Hubble image.

2.
What does the bottom image show?

The close-up Hubble photo at bottom shows a dark, flaring, disk-like structure [off center] bisecting the bright light from the object. The disk is seen edge-on. Although too large to be an accretion disk, this dark, flaring disk may provide indirect proof of the other's existence. Most theories for producing jets require the presence of an accretion disk.

The jets are seen streaming from both sides of the central object. The round, white objects at the lower left and upper right corners are two bright clumps of gas in the jets. The astronomers traced the jets to within 1,000 astronomical units (one astronomical unit equals the Earth-Sun distance, 93 million miles) of the central obscured star. The star ejected this jet material about 30 years ago.